997 the Site of Verrebroek 'Dok'
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THE SITE OF VERREBROEK ‘DOK’ AND ITS CONTRIBUTION TO THE ABSOLUTE DATING OF THE MESOLITHIC IN THE LOW COUNTRIES Mark J Y Van Strydonck1,2 • Philippe Crombé3 • Ann Maes1 ABSTRACT. The wetland site of Verrebroek “Dok” situated in northern Belgium is one of the largest and best dated loca- tions of Mesolithic material in northwestern Europe. Salvage excavations organized since 1992 at this large, unstratified open- air settlement have revealed more than 50 spatially independent artifact concentrations with traces of numerous fireplaces. Single entity dating of charred hazelnut shells from surface-hearths and charcoal from hearth-pits was used to obtain infor- mation not only on the sites duration, but also on the relation between the surface hearths and the hearth-pits. The dates were also used to look at discrepancies between the radiocarbon chronology and the typo-chronology of the lithic artifacts. INTRODUCTION Problems in Dating Stone-Age Settlements on Sandy Soils In a recent article (Crombé et al. 1999) we discussed the problems of absolutely dating Final Pale- olithic and Mesolithic camp sites situated on highly bioturbed sandy soils within the Low Countries. The origin of this dating problem is caused by: 1) a bad or doubtful spatial association of the dated sample and the lithic industry, 2) dislocation caused by bioturbation processes, 3) multiple site occu- pation, and/or 4) inbuilt age of the dating material. The use of single pieces of charred hazelnut shells (CH) from surface-hearths (SH) and single charcoal (CC) fragments from hearth-pits (HP) was rec- ommended as a protocol for future dating projects in the sandy area of northwestern Europe. Between 1998 and now further testing has been done on the Mesolithic site of Verrebroek in view of refining the proposed dating strategy. The results of this project will be discussed in the present paper. SITE DESCRIPTION The site of Verrebroek “Dok” (51°16′00′′N, 4°12′42′′E) is situated in the polder area of Flanders on the extreme east end of a late glacial aeolian sandy dune ridge (ca. 80 km long, ca. 2–3 km wide) that blocks the north part of the Plain of the Flemish Valley. This sandy elevation in the lowlands of Flanders was intensively used during the stone ages, in particular during the Final Paleolithic and the Mesolithic, as can be demonstrated by the presence of numerous sites (Crombé 1998b:95–102). Sites situated on the eastern extremity of this sand ridge, such as the important sites of Melsele “Hof ten Damme” (Van Roeyen et al. 1992; Van Strydonck et al. 1995), Verrebroek “Dok” (Crombé et al. in press) and the recently discovered site of Doel (Crombé et al. 2000), are the only ones that are well-preserved. Peat and alluvial deposits from the river Scheldt have protected these sites against erosion and destruction. Since 1992 the Department of Archaeology of the Ghent University is excavating the site of Verre- broek “Dok”. Actually about 6000 m² have been investigated, corresponding to around 20% of the entire site-surface. The excavations revealed at least 50 artifact concentrations of varying size and composition. The smallest units cover 1–2 m², the largest ones 50–100 m². The majority of units however is smaller than 10–15 m². All excavated units mainly consist of stone artifacts, mostly made of local flint of inferior quality. Some units also yielded a certain amount of exotic raw materials, 1Royal Institute for Cultural Heritage, Jubelpark 1, B-1000 Brussels, Belgium 2Corresponding author. Email: [email protected]. 3Postdoctoral Fellow of the Fund for Scientific Research-Flanders, Department of Archaeology and Ancient History of Europe, Ghent University, Blandijnberg 2, B-9000 Ghent, Belgium © 2001 by the Arizona Board of Regents on behalf of the University of Arizona RADIOCARBON, Vol 43, Nr 2B, 2001, p 997–1005 Proceedings of the 17th International 14C Conference, edited by I Carmi and E Boaretto 997 998 M J Y Van Strydonck et al. Figure 1 Schematic and preliminary draft of the artifact units excavated between 1997 and 1999. 1. hearth-pits; 2. bone clusters; 3. surface-hearths; 4. artifact units; 5. Medieval ditches. such as quartzites and sandstones. Due to unfavorable preservation conditions (Ph-value and oxida- tion degree) organic material is extremely badly preserved on the site. It appears that only carbon- ized organic residue, such as charcoal, hazelnut shells and bones, is preserved. Besides numerous natural features (e.g. tree-falls) three main archaeological features are found at Verrebroek (Figure 1 above): i) surface-hearths (SH), ii) hearth-pits (HP), and iii) pits. Due to later soil processes (e.g. formation of a heather podzol) the SH are no longer preserved as real soil features. Their position can only be deduced by the presence of concentrations of burnt artifacts and ecofacts. Associated charcoal was never observed. SH are always found within the boundaries of artifact concentrations associated with different types of tools (mainly microliths and scrapers) and lithic knapping waste (flakes, bladelets, chips, cores). It thus appears that these SH were used for various purposes (warming, cooking, melting of resin, etc.) and therefore can be interpreted as domestic hearths. On a spatial level it could be observed that all units smaller than 15–20 m² gener- ally display one single SH, whereas larger units posses more SH, usually two but occasionally up to four and even five specimens (e.g. C.14, C.22, C.40, C.44). It is questionable whether the latter are all synchronous (see below). Site of Verrebok “Dok” 999 The HP (12 examples) on the other hand are visible in the soil as black, charcoal rich, three-dimen- sional structures of 0.50–0.70 m in diameter and 0.35–0.50 m deep. Most HP are located within the boundaries of the artifact units, either in the center or near the periphery. Only few pits are found iso- lated from the artifact units. Contrary to the SH, HP have a very restricted distribution as they are only found in the SW part of the excavated area. This part formerly corresponded to one of the high- est grounds of the landscape. The filling of these HP mainly consists of CC fragments with Pinus as the dominant species, but also Quercus and Corylus (Louwagie et al. in press) were found. Lithics and CH only occur occasionally and in very small amounts. So far burnt bone fragments have not been found in HP. In two units (C.22 and C.14) vague traces of shallow pits filled with numerous CH, in some cases associated with CC, were noticed. At present it is not yet clear how to interpret these features, but one cannot exclude that it concerns remains of shallow hearths used for the roasting of hazelnuts or small storage-pits. SAMPLE SELECTION AND TESTING Only single entities were dated. Each dated sample consisted of either one piece of CH from a SH or one piece of CC from a HP. Generally each feature was dated once. Some SH however were dated by more than one sample to obtain a better precision. This was the case for all features that were dated between 1992 and 1994 (Crombé 1998b:20–2) because some of the earliest AMS dates had very large standard deviations. Multiple dating on different CH samples was also done on some extremely large SH (e.g. in units C.17 and C.22) in order to verify their chronological homogeneity. Furthermore some HP were also dated several times for methodological purposes (see below). All dated samples were cleaned using the acid-base-acid method, converted into graphite and mea- sured by AMS. Since some of the CH samples were badly preserved and some dates deviated largely from the bulk of the measurements it was feared that humic acid infiltration from the overlaying peat layer altered some of the CH dates. To clarify this the efficiency of the pretreatment was tested in two ways. At first a duplo sample from unit C.17 was dated after a repeated pretreatment (Table 1). Furthermore a larger CH sample, containing different individual shells, from unit C.22 (SH.2) was pretreated several times and the different residues as well as the soluble fractions were dated (Table 2). These tests demonstrate that the ABA method was sufficient to clean the samples. Judging by the small size of most HP only a limited “old-wood-effect” on the CC was expected. This was confirmed by dating determined branch and trunk wood from HP.66. It must however be admit- ted that anthracologically the term trunk wood is already applied to wood with a diameter exceeding 5 cm (Crombé et al. 1999). Dating different wood species within HP.91 revealed that the HP were, as expected, most probably only used once within a limited period and that no intrusive or residual CC was present in the pits. Two CC samples (UtC-3450: 8700 ± 100 BP from unit C.6 and UtC-3443: 7700 ± 100 BP from unit C.10) were rejected because soil analysis indicated that they originate from presumed natural pits. The above tests show that the CH as well as the CC samples are reliable and do date respectively the SH and the HP so that there is a close chronological connection between the 14C event and the human event (Van Strydonck et al. 1999). 1000 M J Y Van Strydonck et al. Table 1 Radiocarbon dates from Verrebroek “Dok” 14C age δ13C Cultural Unit Structure Referencea (yr BP) (O) Sample materialb association C.1 SH. UtC-3915 9110 ± 65 −23.0 CH Beuronian A C.2.1 SH.